Electro-optical device



J. A. O'CONNELL I ETAL 3,039,005

ELECTRO-OPTICAL DEVICE June 12, 1962 -2 sheets sheet 1 Filed April 8,1960 FIG. 2

Fiat

TIME

FIG. 3

INVENTORS JAMES A. OCONNELL BERNT NARKEN J1me 1962 J. A.'O'CONNELL ETAL3,039,005

ELECTRO-OPTICAL DEVICE 2 Sheets-Shet 2 Filed April 8, 1960 3,039,005ELEiJTRiP-OPTICAL DEVICE James A. OConnell and Bernt Narken,

International Business Machines Cor- New York, N.Y., a corporation ofNew York Filed Apr. 8, 1960, Ser. No. 21,016

Claims. (or. 250 213 N.Y., assignors to poration,

In a known optical latch circuit, sometimes called an electroluminor(EL) in electrical series the EL, which then through PC at its lowresistance level, A somewhat similar a light output which decays duringa standard delay maintained by the non-saturating feedback.

optical latch or near-latch is merely an array of similar devicesforming a display, it of turning selected optical Luminant turn-on haslong been recognized as inherent in the optical latch. Luminantturn-off, however, has PC configuration quenching of the The phenomenonof photocurrent undershoot has been recognized and explained as typicalof cadmium selemde photoconductors.

at the shaded light level. Photoourrent undershoots; that is,photocurrent drops below the predicted level.

It is the object of h by means of a longer duration light control pulsefrom the same source.

It is another specific object of the invention to provide means fordarkening a non-saturating feedback optical device selectively for acertain delay period under con- Summary 3,039,095 Patented June 12, 1962In] requires a luminor and a PC electrically in series and opticallycoupled for feedback. The PC must exhibit photocurrent undershootcharacteristics when illuminated by a luminant control pulse for aduration greater than device is active, and the high energy currentconducting electrons and holes recombine, sweeping the conduction Thecurrent carrying capacity of the PC immediately after the light isrmoved is thus swept below the level at Which its luminor normally holdsit. This photocurrent undershoot luminor and the device goes olf. The

tion capability below that necessary nance.

The most significant feature of the invention is the extinguishing of anoptical feedback device as a result of a control pulse of light. thisfeature is that a single light G. l is a schematic diagram of apreferred embodiment of the invention.

FIG. 2 is a chart of characteristics of the embodiment of FIG. 1.Luminance of input and output and series resistance are shown as afunction of time.

FIG. 3 is a partially sectioned diagrammatic view of a display using amultiplicity of storage display elements according to FIG. 1.

FIG. 4 is a schematic diagram of a limited or slow feedback embodimentof the invention.

G. 5 is a chart of characteristics of the embodiment of FIG. 4.

FIG. 6 is a structural diagram of the FIG. 4 embodiment.

FIG. 7 is a schematic diagram of a neon-photoconductor embodiment of theinvention.

FIG. 8 is a chart of characteristics of the embodiment of FIG. 6.

A high feedback optical latch has sometimes been called the optron;limited feedback opcalled the persistron. Devices acproducing acontinuous light output; be latched on by a short light input andunlatched by a subsequent long duration light input. The limitedfeedback persistron input and persists on for a standard time durationbefore going off;- the limited feedback normally,

turns it on again.

FIGS. 1 and 2 demonstrate a saturating feedback optical device accordingto the invention. PClll is con- FIG. 1.

aosacce nected to 200 volt 1000 cycle power source 12 and ELl3 which ispositioned for saturating optical feedback with PC11. When in the dark,PCll provides a high resistance in the order of 500 megohms.Footcandle-second timer 14 and light source 15 produce accurately timedoptical pulses which are coupled to PCll to reduce the resistance ofPC11 to the order of 500 kilohms. The voltage division across. the EL-PCseries circuit is thus altered to provide firing voltage to EL13. The BLis optically coupled to PC11 so that after light source 15 isextinguished the EL maintains illumination of the PC for low resistanceand the EL remains luminant. The characteristics of the circuit elementsused and the phys ical configuration are maintained so that opticalfeedback is sufficient to maintain the optical device in the latchedcondition.

P011 is a cadmium selenide PC of the type which exhibits photocurrentundershoot'when brightly illuminated and then suddenly shaded to anintermediate light level. Cadmium selenide with copper or silveracceptors and chlorine donor levels is preferred. Other cadmium selenidephotoconductors are generally acceptable. Cadmiurn sulfide and other PCcompositions which do not exhibit appreciable photocurrent undershootare generally ineffective at room temperature. At the feedbacksaturation level of light, photon energy creates electron-hole pairs atthe same rate that previous electron-hole pairs recombine. The forbiddenband between conduction band and valence band is stable, with holesarrayed along the valence band and electrons in the current band.Additional light from the input creates additional electronhole pairs.As the input light is shaded, the mobile electrons recombine rapidlywith the holes, temporarily sweeping the conduction band completely freeof carriers, causing photocurrent undershoot.

Timer 14 causes light source 15 to illuminate PCll with a short durationstandard luminance input pulse 21 (FIG. 2). Due to internal delay, PC11cannot attain sufficient energy levels to cause photocurrent undershootwhen subsequently shaded. As the resistance 24 of PC11 diminishes as aresult of the light input, the voltage across EL13 rises above thefiring threshold, causing a minimal luminant output 25 which is fed backto PClli. The regenerative feedback during the short input pulse issuflicient to latch the optical device for optical output 22. Outputluminance stabilizes at the level of point 26.

When timer 14 causes light source 15 to illuminate PC11 with a longerduration light pulse 23 of standard luminance, carriers in the PC attainhigh energy levels during the light input. Lurninant output rises to thelevel of point 27. When the light input 23 suddenly is shaded, thecarriers sweep the conduction band of PC11 so free of carriers that thephotocurrent undershoot phenomenon occurs. Photocurrent undershoot, ormore specifically the high resistance 28 which accompanies it, lowersthe effective potential across EL13 to a value below the cutoffpotential whereupon it is extinguished and the latch condition is lostas luminant output falls below point 29 which is below saturation region20.

FIG. 3 illustrates a display made up of an array of optical elementseach corresponding to the schematic of Four photoconductive elements 30are shown arrayed over conductive layer 31 on glass plate 32. Asimilarly dimensioned glass plate 33, equipped with a conductive coating34, sandwiches phosphor layer 35 between itself and the array ofphotoconductors. Contacts 36 connect power source 3'7 across ELPCsandwich, with elemental areas of BL phosphor layer 35 in electricalseries with each of the PCs 30 with optical coupling to the PCs. Eachphotoconductor and the electroluminescent phosphor in its immediatevicinity form the optical element corresponding to the schematic inFIG. 1. Timer 38 and light source 39 are adapted to selectivelyilluminate the various photoconductor elements for a short duration toset them or for a longer duration to extinguish them.

A flying spot scanner or cathode ray tube light source can set acomplete picture into the optical elements for continuous display on afast scan. To update the display, areas to be darkened are illuminatedduring a slow scan for erasure. Alternatively, the entire display may beblanked by a relatively long unfocussed illumination prior to each scan.Another method is a high intensity erasing scan from one sourceimmediately followed by a low intensity writing scan.

H68. 4 and 5 demonstrate schematically a limited feedback optical deviceand its parameters. PC41 is low dark resistance value in the order of lto 2 megohms by shunt resistance 42; one side of the PC and resistanceis connected to power source 43. The other side of the PC is connectedto EL44 which is positioned for limited optical feedback with PC41.Timer 4S and light source 46 control input illumination. FIG. 4 differsfrom H6. 1 only in the addition of variable shunt resistance 42 which isadjusted to place the device just over saturation line 51 (PEG. 5) fornormal light output 52.

PEG. 6 is a structural diagram of the device of FIG. 4. Glass plate 61is coated with a transparent conductive electrode forming layer 62 oftin oxide. EL layer 63 covers layer 62 and is in turn covered byinsulator layer 6% except at a small hole 65. PCed is deposited overinsulator 64 and through hole 65 where it makes series connection withEL layer 53;. Tin oxide layer 67, of bulk calculated to produce darkresistivity of the order of l to 2 mcgohrns, is positioned over PC66,making contact with EL63 through hole 65. Connection of power supply 68makes the device correspond to the schematic diagram of FIG. 4.

'Jhen PC41 is illuminated by a light pulse 53, carriers in the PC attainhigh energy levels. When light pulse 53 ends, the high energy carrierssweep the PC conduction band so free of carriers that resistance 54mornentarily rises above the saturation region 51 to shade the deviceand provide an interruption in the otherwise continuous high intensityoutput. The device persists off only momentarily, for as the PC returnstoward its normal resistance value it returns to saturation region 51.Feedback recurs and output reappears.

FIGS. 7 and 8 demonstrate schematically a neon-PC embodiment analogousto the ELPC embodiment of P16. 1. PO71 forms a series resistance dividerwith neon 72 and power source 73. The resistance of the divider variesfrom intermediate to low according to optical inputs to PC71 from lamp74 as controlled by timer '75. Neon 72 is in series with the resistancedivider network and is adapted to fire when PC'71 is illuminated. Asneon 72 fires, it illuminates PO71, to maintain the resistance dividervoltage drop below neon cutoff level and to latch the optical device on.Megohm range resistance 7 6 across neon 72 establishes firing potential.

PC71 is a cadmium selenide photoconductor with optical undershootproperties similar to that of P011 in FIG. 1. The device turns on as aresult of a short duration pulse; it turns off at termination of alonger duration pulse which causes photocurrent undershoot.

Timer '75 causes light source 74 to illuminate PC71 with a shortduration standard luminosity input pulse 81 (FIG. 8). PC71 cannot attainsufficient energy levels to cause photocurrent undershoot whensubsequently shaded. As the resistance of PC71 diminishes during pulse81, the voltage across neon "iZ'is raised. Light output 82; occurs whenthe voltage rises above the firing potential (above region Regenerativefeedback latches the device on for output 82.

When timer 75 subsequently causes light source 74 to illuminate PC71with a longer duration light pulse 83 of standard luminance, carriers inthe PC attain hig energy levels which, when the light input suddenly isshaded, sweep the conduction band of PC71 carriers that photocurrentundershoot occurs. The high resistance 84 Which accompanies undershootlowers the effective voltage across neon 74 to a value below the cutoffpotential (below region 85) whereupon the device goes dark and the latchThe standard light sources in FIGS. 1, 4 and 7 may generally be ordinarylamps of reasonable intensity. The timers may be electronic timershaving short and long durations selectable by external stimuli. Sincephotocurrent undershoot is a function of both duration and intensity ofilluminavarious fractional combinations of footcandles and seconds areeifective. The combination of lamp and footcandle-second timer. Two

the footcandle-second timera minimum footcanclle-second light pulse tocause footcandle-second light pulse and destroy feedback.

so free of the device.

4. An optical feedback device according to claim 1 photoconductor iscadmium selenide With acceptors in the group consisting of copper andsilver chlorine donors.

5. A limited feedback optical device according to claim 1, comprising111 addition resistance means to bias the deing a standard time durationtransparency electrical insulator layer adjacent said phossaid insulatorhaving an opening,

conductor layer and a source of power.

References Cited in the file of this patent UNITED STATES PATENTSOrthuber et al. June 3, 1958 azan July 21, 1959 OTHER REFERENCES ThePhotoelectric Cell, The General Electric Review, July 1954, pages 28,29, 30 and 31.

Analysis of Photoconductivity Applied to Cadmium Sulphide TypePhotoconductors, Journal of Physics and Chemistry of Solids, volume 1,No. 4, 7, pages 234- 248

